1. Field of the Invention
The present invention relates to a valve drive apparatus, in which a shaft is driven through a speed reducing mechanism to rotate a valve integrally with the shaft.
2. Description of Related Art
Japanese Unexamined Patent Publication No. 2009-150252A teaches a valve drive apparatus, which drives a shaft through a final gear of a speed reducing mechanism to rotate a valve fixed to the shaft.
The valve drive apparatus of Japanese Unexamined Patent Publication No. 2009-150252A will be described with reference to FIGS. 10A and 10B.
The valve drive apparatus of Japanese Unexamined Patent Publication No. 2009-150252A includes the shaft 102, the final gear 103 of the speed reducing mechanism and a valve opening degree sensor 106. The shaft 102 is rotated integrally with the valve. The speed reducing mechanism increases a rotational torque of the electric motor. The valve opening degree sensor 106 senses an opening degree of the valve (shaft 102) and includes a magnet 104 and a magnetic sensing device 105.
The final gear 103 is joined to the shaft 102 through a final gear support portion 107, which is formed integrally with the final gear 103, and an output of the speed reducing mechanism is conducted to the shaft 102.
In the valve opening degree sensor 106 of Japanese Unexamined Patent Publication No. 2009-150252A, the magnet 104 is installed to the final gear support portion 107, and the magnetic sensing device 105 is installed to a housing 111 (stationary member).
When the final gear 103 is rotated, a distance and a positional relationship between the magnet 104 and the magnetic sensing device 105 are changed. Thereby, as shown in FIG. 10B, the valve opening degree sensor 106 outputs a signal of low (Lo) corresponding to an OFF state or a signal of high (Hi) corresponding to an ON state. That is, the valve drive apparatus of Japanese Unexamined Patent Publication No. 2009-150252A senses the open/close state of the valve with the valve opening degree sensor 106 of a switch type.
The connection between the shaft 102 and the final gear support portion 107 receives an output torque of the electric motor, which is amplified with the speed reducing mechanism, and also a force, which is applied to the valve and is conducted to the connection through the shaft 102.
In this way, the strong rotational torque is applied to the connection between the shaft 102 and the final gear support portion 107.
When the connection between the shaft 102 and the final gear support portion 107 is damaged by any possibility (thereby enabling free rotation of the shaft 102 relative to the final gear support portion 107), it is required to sense the damage (failure) at the connection between the shaft 102 and the final gear support portion 107.
However, in the prior art technique described above, the magnet 104 is installed to the final gear support portion 107, so that even in the state where the connection between the shaft 102 and the final gear support portion 107 is damaged, the sensed result of the valve opening degree sensor 106 with respect to the rotation of the final gear 103 becomes normal.
That is, in the prior art technique, the damage of the connection between the shaft 102 and the final gear support portion 107 cannot be sensed based on the sensed result of the valve opening degree sensor 106.
As discussed above, the valve opening degree sensor 106 of Japanese Unexamined Patent Publication No. 2009-150252A is of the switch type, and thereby the sensed result of the valve opening degree sensor 106 becomes the low (Lo) in both of a full close position and a full open position of the valve, in which the magnet 104 is placed distant from the magnetic sensing device 105, as indicated in FIG. 10B.
That is, the valve opening degree sensor 106 of Japanese Unexamined Patent Publication No. 2009-150252A provides the same sensed result, i.e., the low (Lo) in both of the full close position and the full open position of the valve.
Thus, the sensed result of the valve opening degree sensor 106 becomes normal even in a case where the valve is placed in the full close position despite of a fact that the valve is supposed to be in the full open position or in a case where the valve is placed in the full open position despite of a fact that the valve is supposed to be in the full close position. Such an incident occurs in, for example, a case where a wiring error of the electric motor (e.g., a case where a cathode and an anode are erroneously reversely connected in the electric motor) occurs to cause rotation of the valve by the electric motor to an opposite side, which is opposite from a demanded side, or in a case where the valve is rotated to an opposite opening degree, which is opposite from a demanded opening degree, by an external force.
That is, according to the prior art technique, even when the open/close state of the valve is reversed from its demanded state, such a problem cannot be sensed based on the sensed result of the valve opening degree sensor 106.